The morphology, orientation, and position of the acetabular and femoral components of the hip joint are important determinants of its function. The success of the native hip is based on design parameters of these components that, within a normal range of motion, avoid bony and soft tissue impingement. The native acetabulum is not perfectly hemispherical, and its rim is not simply a circle but rather a 3D shape with a series of prominences and depressions. Similarly on the femoral side, the junction between the head and the neck is not perfectly circular.
Modern hip resurfacing implant designs are based on a hemispherical acetabular cup with a level circular rim. On the femoral side, variable proportions of a sphere have been adopted by different implant manufacturers. It has been shown that acetabular cups that overhang the rim of the acetabulum are associated with impingement, early loosening and accelerated wear. This overhang would be expected when using a hemispherical cup, as part of its rim will be prominent in areas of low acetabular rim profile.
Determination of the 3D position of the acetabulum in the pelvis and the femoral head in relation to the femoral neck has continued to be a challenge. Antero-posterior acetabular position, which is not possible to quantify on plain radiographs, affects the function of the hip muscles. It may also be associated with impingement. Femoral head-neck relationship is similarly difficult to quantify.